"Any period using sterling would be deeply damaging to a newly independent Scotland, meaning it could not create its own economic, monetary and fiscal policies, in turn meaning that it might well have less real power than Holyrood now because the reality would be that all power would lie in London. What is more, under EU rules it has to have its own currency to apply for membership. So it would fail there too. If the SNP is serious about independence there is only one option available to it, and that is its own currency from day one. Otherwise it will be trapped in dependency forever, which is the exact opposite of what it says it wants."
https://www.taxresearch.org.uk